Silicon nitride is a material well balanced in strength, fracture toughness and resistances to-corrosion, abrasion, thermal shock and oxidation, etc., and has become the center of attraction recently as an engineering ceramic for structural members at room temperature and high temperature. However, in order to use silicon nitride ceramics in fields requiring high reliable materials, for example, in automobile parts, etc., it is indispensable to further improve the fracture toughness of the ceramics to overcome the brittleness and increase the strength thereof. An increase in the strength of a ceramic that is a polycrystalline material has heretofore been contrived by refining the individual crystal grain thereof, but this method lowers the fracture toughness of the material which makes it more brittle. Japanese Patent Publication No. 265173/1987 discloses a technique for improving the fracture toughness of a ceramic material by combining a silicon nitride matrix with silicon carbide wherein the whiskers are dispersed in the matrix. It is thought that according to the above-mentioned method the fracture toughness of the ceramic material is improved as the cracks, which may progress and expand during the fracture, are deflected by the whiskers or because extraction or crosslinking of the whiskers takes place. The fracture toughness of the ceramic material is therefore improved by the combination with the whiskers. However, it is difficult to completely remove the agglomerates of the whiskers by a mechanical means and when the size thereof is of the order of 1 to 10 .mu.m, such whiskers or agglomerates, like the coarse grain, form breaking points, thereby decreasing the strength of the ceramic material. Such a decrease in strength is also observed in a composite material of long fibers. In addition, the composite material of the particle dispersion type that is formed by mechanically mixing the particles having a diameter on the order of microns with the matrix and firing the mixture cannot exhibit the remarkable compounding effect of the dispersed particles in both strength and toughness.
According to the conventional process for producing a composite sintered body such as a process in which silicon nitride, acting as the host phase, is mechanically mixed with silicon carbide in a dispersed phase to form a mixture which is then sintered, it is substantially impossible to form the dispersed particles having a sufficiently small average particle size in the sintered body, since the average particle size of the raw powder material is of the order of one micron or at least several hundred nanometers. Accordingly one cannot expect to increase the strength of the composite sintered body by the above conventional process.
Further Japanese Patent Laid-Open No. 159256/1988 discloses a process in which silicon carbide particles having an average particle size of 1 .mu.m or smaller are homogeneously dispersed in silicon nitride particles and the silicon nitride particles are subjected to grain growth to form columnar crystals. However, even with the aforementioned composite sintered body of silicon nitride-silicon carbide, a lower proportion of silicon carbide tends to form silicon nitride of columnar crystal, thus minimizing the improvement in strength in spite of some improvement in fracture toughness, whereas a higher proportion of silicon carbide suppresses the formation of silicon nitride of columnar crystal, thereby lowering the fracture characteristics, thereby decreasing the strength.
Under such circumstances, in order to obtain a composite material with fine dispersed particles as a sintered body, it is effective to adopt a process for producing dispersed particles in situ during the sintering by compositing the raw powder materials themselves. For example, as disclosed in Japanese Patent Laid-Open No. 160669/1990, it is possible to cause further refined silicon carbide particles to precipitate by heating an organosilicon compound consisting essentially of silicon, nitrogen and carbon in a non-oxidative gas containing ammonia, adding a sintering aid to the resultant amorphous composite powders, and sintering the composite powders to crystallize silicon carbide in situ during the sintering.
As described above, for the purpose of increasing the strength and toughness of a silicon nitride sintered body, it is effective to produce a composite sintered body of silicon nitride-silicon carbide from a composite material having a sufficiently small size. It has been found, however, that according to the conventional process wherein amorphous composite particles consisting essentially of silicon, nitrogen and carbon as the raw powder materials are sintered as such, the sintering is accompanied with the local formation of a coarse aggregated structure of silicon carbide with an extremely large particle size of 10 .mu.m or larger, which form breaking points and causes the resultant sintered body to break at a level lower than the strength inherent in the material of the sintered body.
Specifically, the sintering of amorphous composite particles consisting essentially of silicon, nitrogen and carbon brings about the crystallization of silicon nitride as .alpha.-phase and silicon carbide as .beta.-phase and final densification in the high-temperature region, but is accompanied by the gas phase-solid phase reaction as represented by the following scheme, thereby locally forming a coarse aggregated structure of silicon carbide:
(1) formation of carbon monoxide gas by the reaction of the free carbon in the powder with the oxygen in the oxidized layer of the powder surface or in the oxide of the sintering aid: EQU C(solid)+O(solid).fwdarw.CO(gas),
(2) formation of silicon the dissociation of the siliconnitrogen bond in the powder: EQU Si.sub.3 N.sub.4 (solid).fwdarw.SiC(solid)+N.sub.2 (gas),
(3) formation of silicon carbide by the reaction of the resultant carbon monoxide gas with silicon: EQU CO (gas)+Si(solid)+O.sub.2 (gas), and
(4) formation of silicon carbide by the reaction of the resultant carbon monoxide gas with silicon: EQU CO+Si.sub.3 N.sub.4 (solid).fwdarw.SiC(solid)+NO.sub.2 (gas).
According to the conventional process, therefore, it has been extremely difficult to simultaneously improve the strength and toughness of a silicon nitride ceramic, since an increase in strength by refinement of the structure lowers the fracture toughness and conversely, an improvement in the fracture toughness by allowing large columnar crystals to exist by the addition of whiskers or grain growth of silicon nitride lowers the strength. It has, therefore, been a serious subject to reconcile strength with toughness for silicon nitride ceramics.
In view of the above, an object of the present invention is to provide a composite silicon nitride sintered body excellent in both strength and fracture toughness wherein the formation of defects due to coarsening of silicon carbide is suppressed, and a process for producing the sintered body.